1. Field of the Invention
The present invention relates to an optical switching device for performing the setting and switching of optical routes between a plurality of input and output ports, and in particular, to an optical switching device suitable for the use in constructing an optical communication system which is capable of processing a signal light containing large volume information while maintaining an optical state thereof, without converting the signal light into electrical signals.
2. Description of the Related Art
In a photonic network in a metropolitan access area, a network configuration is frequently modified by the exchange of lines and signals and the switching of routes in an add/drop multiplexing (ADM) node or the like. In a repeating stage in the current network, there is often adopted a configuration in which an optical signal is once converted into an electrical signal and then is converted into an optical signal, to thereby perform the signal switching.
However, in years to come, it is predicted that such an add/drop multiplexing node will be replaced by a dynamic optical add/drop multiplexing (OADM) node which separates only a desired wavelength while maintaining a state of optical signal, an optical cross connecting (OXC) node which performs the switching of input and output routes in a wavelength basis while maintaining a state of optical signal or the like. Further, in the further next generation, it is anticipated that, in order to improve the line usage efficiency, it would be necessary to prepare a function of dividing optical signals into fixed length frames, to perform the processing of the signal exchange and of the route switching (hereunder, to be referred generically to as the optical burst signal processing) in the frame basis while maintaining a state of optical signal.
As one of important optical components necessary for realizing the above, there is an optical switch which includes a plurality of input ports and a plurality of output ports. As one example of a conventional multi-input and multi-output port type optical switch, literatures 1 to 6 shown in the followings each discloses a technology relating to an optical switch module using optical deflection elements.
Literature 1: Japanese Unexamined Patent Publication No. 2002-318398
Literature 2: Japanese Unexamined Patent Publication No. 2003-185984
Literature 3: Japanese Unexamined Patent Publication No. 2000-114629
Literature 4: Japanese Unexamined Patent Publication No. 2002-269892
Literature 5: Japanese Unexamined Patent Publication No. 7-212315
Literature 6: Japanese Unexamined Patent Publication No. 10-228007
To be specific, as shown in FIG. 5 for example, the conventional optical switch module using the optical deflection elements comprises an incident side optical waveguide section 101, a collimating section 102, an incident side optical deflection element section 103, a common optical waveguide 104, an emission side optical deflection element section 105, a collecting section 106 and an emission side optical waveguide section 107. In the optical switch module 100 of such a configuration, for example in the case of switching a route of a signal light given to one end of an optical waveguide 101a-i (i=1, 2, . . . , n), which corresponds to an input port #1i, in the incident side optical waveguide section 101, to an optical waveguide 107a-j (j=1, 2, . . . , n), which corresponds to an output port #2j, in the emission side optical waveguide section 107, the signal light emitted from the other end of the optical waveguide 101a-i is converted into a parallel light in a collimator 102a-i to be given to an incident side optical deflection element 103a-i. To the incident side optical deflection element 103a-i, a voltage according to a position of the optical waveguide 107a-j being the output determination is applied from a control circuit (not shown in the figure), so that a traveling direction of the signal light from the collimator 102a-i is deflected. Then, the signal light polarized by the incident side optical deflection element 103a-i travels straight through a free space in the common optical waveguide 104 to reach an emission side optical deflection element 105a-j. In the emission side deflection element 105a-j, the traveling direction of the signal light is deflected according to a position of a collective lens 106a-j by the voltage application from the control circuit, so that the signal light is collected by the collective lens 106a-j to be given to the optical waveguide 107a-j. As a result, the route of the signal light given to the input port #1i is switched to the output port #2j. 
Incidentally, herein, the configuration using the optical deflection elements is shown as one example of multi-output ports type optical switch. However, other than such a configuration, there is also known, for example, the configuration utilizing a semiconductor optical amplifier (SOA), a micro electro mechanical system (MEMS) mirror or the like.
In an apparatus for performing the optical burst signal processing utilizing the conventional optical switch module as described above, when the exchange of signal lights and the route switching in the frame basis are to be realized, since it is required to perform the switching processing at least at a time dimension (for example, a microsecond dimension) smaller than a millisecond dimension, the route switching needs to be performed sequentially on the frame signal lights input from the various routes. However, there is caused a problem in that, when such processing is performed, there may be an influence by a difference in input power to the optical switch module, a loss difference between the input and output ports of the optical switch module or the like, so that a variation occurs in output power values of the output frame signal lights, thereby affecting the performance of error-free reception in an optical receiver.
In order to solve the problem relating to the optical switch module control in the optical burst signal processing as described above, the applicant of the present invention has proposed an optical switching device of a configuration as shown in FIG. 6 for example (refer to Japanese Patent Application No. 2005-102763). In the optical switching device according to the invention in this prior application, a reference light Lb whose wavelength is set to be outside a wavelength band of a signal light Ls which is given to each of input ports #11 to #1n of an optical switch module 100, is given from a reference light source 111 to each of the input ports of the optical switch module 100 via each of wavelength multiplexing couplers 112-1 to 112-n, and the reference light Lb contained in the light output from each of output ports of the optical switch module 100 is extracted by each of wavelength separating couplers 113-1 to 113-n, so that the power thereof is monitored by a reference light power monitor 114. Thereby, a loss in the optical switch module 100 is monitored and the monitoring result is stored in a control circuit 115. Then, when a route setting control and a variable attenuating control are performed on the input signal light, a control parameter for the optical switch module 100 is calculated based on the stored information and the input signal light power monitored by an input signal light power monitor 110, so that the optical switch module 100 is controlled in accordance with the control parameter. As a result, a power variation in the frame signal lights output from respective output ports #21 to #2n is suppressed.
The invention in the prior application as described above is significantly effective as a specific optical switch control technology for resolving the variation in output optical power in the optical burst signal processing at the time dimension smaller than the millisecond dimension. However, even in this invention in the prior application, there remains a problem as shown in the following which is caused by the wavelength dependence of the optical switch module.
Namely, in the device configuration shown in FIG. 6, the propagation direction of the signal light Ls (solid lined arrow in the figure) is same as the propagation direction of the reference light Lb used for grasping the characteristic of the optical switch module 100. Therefore, in order to prevent the reference light Lb from not entering into the output light of the optical switching device, it is required that the wavelength of the reference light Lb is set to be outside the wavelength band of the signal light Ls, to be separated from the signal light Ls by the wavelength separating coupler 113. Therefore, in the case where the characteristic of the optical switch module 100 includes the wavelength dependence, there is a problem in that an error occurs in the information (calibration data) obtained by using the reference light Lb whose wavelength is different from that of the signal light Ls.
Incidentally, in the specification of the prior application, there is disclosed one example in which a correspondence relationship between a power characteristic of the monitored reference light and a power characteristic of the signal light is previously stored in order to correct the error due to the difference between the wavelength of the signal light and that of the reference signal. However, it is difficult to perform the error correction with sufficient precision in the case where the characteristic of the optical switch module 100 is changed due to, for example, the temperature variation, the degradation with time or the like. Further, there is disclosed the case where the reference light Lb whose wavelength is same as that of the signal light Ls is supplied. However, in this case, it is necessary to dispose separately an optical switch for switching the supply determination of the reference light Lb according to whether or not the optical route is set, and also a high-speed operation at least at a time dimension smaller than the millisecond dimension is required for such an optical switch. Therefore, the configuration becomes complicated and also the cost thereof rises.
Further, in the device configuration shown in FIG. 6, the relatively large loss occurs in the optical switch module 100, but this loss is not compensated in the optical switching device. Although it becomes possible to compensate for the loss as described above by disposing an optical amplifier in the optical switching device, a transient response in the optical amplifier becomes problematic only by simply disposing the optical amplifier. To be specific, as shown in FIG. 7 for example, if the burst signal light of waveform as shown in the lower left of the figure is input to the optical amplifier disposed in the optical switching device, at the rising time of the burst signal light, a surge as shown in the lower right of the figure occurs in the output waveform of the signal light amplified by the optical amplifier, to adversely affect on the various devices connected to the downstream of the optical switching device. Therefore, the loss compensation considering the transient response of the optical amplifier is the subject to be solved.